Hydraulic and mechanical presses are used, among other things, for bending long metal sheets or plates. The metal being bent is called a workpiece. Bending is accomplished by placing the workpiece between a male die and a female die and bringing the dies together with sufficient force to bend the workpiece. One die is on a moveable member called a ram and the other is fixed to the bed of the press. The ram and bed of the press are designed very heavy structurally so as to provide rigidity to the dies.
Except for bending very thin material, most long sheet or plate bending is accomplished by the "air bending" concept which is also called three-point bending. In air bending, the male die forces the workpiece over the two contact points of the female die and the angle of the bend is established by how far the nose of the male die enters the female die with the workpiece between them. Very small variations in the distance the male die penetrates will cause significant variations in the angle of the bend of the workpiece. For example, for a one-inch female die opening, a difference of 0.02 inches is how far the nose of the male die enters the female die will cause a difference of 2.2 degrees in the angle of the bend.
Particularly when bending long workpieces, tremendous force, ranging from 3 to 50 tons per foot of length for steel, is needed to bend the workpiece. This great force causes the ram and bed members of the press to deflect much as a loaded beam will deflect. Although the actual amount of deflection may be relatively small, as indicated above, it causes the angle in the workpiece to vary along its length, and since loading beams deflect more in their centers than toward their edges, the workpiece bent without some type of deflection compensation will have an unequal angle of bend along its length and will be underbent or "bellied" toward the center. Such bend angle error is many times unsatisfactory. For example, if a bent plate is to be welded to an adjacent plate that is flat, the "bellied" edge creates a very difficult fit-up and welding problem.
Many devices have been used to compensate for press deflection. Probably the most common way to deal with deflection is to shim the die progressively between its center and its ends. Shims are time consuming to install because they must be placed by hand, and shims do not produce predictable and reproduceable results because shimming is largely based on a trial and error procedure.
Another technique to avoid adverse consequences of deflection is to crown the ram or bed of the press, or to use an intermediate die holder with varying thickness that approximates the expected deflection of the press when force is applied to the workpiece. The problem with crowning is that the crown compensates for deflection for only one load condition, and for every other load condition a problem is caused by too much or too little crowning.
Another technique to deal with deflection is to use a number of individual transverse wedge blocks that can be inserted between the die and a bolster. These wedge blocks can be inserted under the die individually to a position that compensates for the deflection of the press. As with shims, individual wedge blocks must be individually positioned by hand, and their placement is based largely on trial and error, and is time consuming.
Other techniques to deal with press deflection are to use a die holder of trapped elastic materials such as rubber or plastic, or to support a die on fluid such as oil that supports the die through a hydraulic cylinder or a diaphragm. These methods require complex and expensive equipment which requires frequent maintenance and replacement.